1. Field of the Invention
The present invention relates to an optical pickup device for use in an optical information recording/reproducing apparatus which uses a light beam to write and read an information signal into and from an optical information recording medium such as an optical disk.
2. Description of the Related Art
An optical pickup comprises an irradiation optical system including an objective lens and an optical detecting system for focusing a light beam irradiated from a light source on a sequence of pits, a track or the like formed spirally or concentrically on an information recording surface on one side of an optical disc such as CD (Compact disc), CD-ROM and DVD (Digital Versatile Disc) to form a spot thereon, and read recorded information such as music and data from return light reflected back from the information recording surface of the optical disc, or for writing recording information on a track or the like.
In the optical pickup, so-called focus servo and tracking servo for an objective lens are essential for securely writing information on an optical disc or securely reading information from the optical disc. The tracking servo control is a position control in a radial direction on an optical disc with respect to a track, over which the objective lens is positioned, for irradiating a light beam to a recorded location (for example, a track) on an information recording surface of the optical disc at all times. The focusing servo control is a position control in the axial direction of the objective lens for minimizing a positional error in the axial direction (focusing direction) of the objective lens with respect to a focused position of the objective lens such that the light beam is converged at the recorded location in the form of spot.
Known focusing servo control methods include, by way of example, a spot size method which divides light into two optical paths in an optical system of return light, focuses one of light beams on a front detector while focuses the other light beam on a rear detector, and compares the sizes of light spots on the front and rear detectors, and an astigmatic method which employs a cylindrical lens, a parallel flat plate and so on positioned in an optical system of return light, receives the return light on a quadrant detector, and detects the shape of a light spot on the detector.
The spot size method requires an optical pick up of a large size as a whole since return light must be divided, whereas the astigmatic method readily calculates a tracking error signal for a tracking servo control in accordance with a DPD (Differential Phase Detection) scheme since a defocused state is detected at a high sensitivity and a quadrant detector is employed for light detection. The astigmatic method is also advantageous in that it is readily applied to a three-beam based optical pickup which uses three light spots since an optical pickup of smaller size can be employed.
An example of conventional optical pickup device using the astigmatic method is illustrated in FIG. 1. A light beam from a semiconductor laser 1 transmits a polarizing beam splitter 3, a collimator lens 4 and a quarter wavelength plate 6, and is focused by an objective lens 7 on an optical disc 5 positioned near the focus of the optical lens 7. The light beam is thus transformed into a light spot SP on a sequence of pits (track) on an information recording surface of the optical disc 5.
Light reflected back from the optical disc 5 is converged by the objective lens 7, transmits the quarter wavelength plate 6 and the collimator lens 4, is redirected by a polarizing beam splitter 3, and passes through a cylindrical lens 8 which applies astigmatism to the light. The resulting light forms a light spot SP near the center of a quadrant photodetector 9 which has a light receiving surface divided into four by two line segments which intersect perpendicularly in a track extending direction and in a disc radial direction.
The cylindrical lens 8, as illustrated in FIG. 2, is positioned on an optical path of the return light such that its central axis extends at an angle of 45° with respect to a direction in which the track of the disc 5 extends, so that the return light forms a line image M, an image plane B (hereinafter called the “minimum scattered circular image plane) on which a light beam becomes circular (minimum scattered circle) in the optical system applied with astigmatism, and a line image S. Therefore, the cylindrical lens 8 irradiates the quadrant photodetector 9 with a circular light spot SP as illustrated in FIG. 3A on the minimum scattered circular image plane B when a light beam converged on the recording surface of the optical disc 5 is focused, and irradiates the quadrant photodetector 9 with an elliptic light spot SP extending in a diagonal direction of the four-divided light receiving surface as illustrated in FIG. 3B or 3C when the light beam is defocused (when the optical disc 5 is too far (b) or too near (c) from the optical disk 5 illustrated in FIG. 1).
The quadrant photodetector 9 opto-electrically transduces a portion of the light spot irradiated to each of the four light receiving surfaces into an electric signal in accordance with its light intensity, and supplies the electric signals to a focus error detector circuit 12. The focus error detector circuit 12 performs a predetermined operation based on the electric signals supplied from the quadrant photodetector 9 to generate a signal (hereinafter called the “focus error signal” or FES) which is supplied to an actuator driving circuit 13. The actuator driving circuit 13 supplies a focusing driving signal to an actuator 15. The actuator 15 moves the objective lens 7 in a focusing direction in response to the focusing driving signal. In this way, the focus error signal is fed back to control the position of the objective lens.
As illustrated in FIG. 4, the quadrant photodetector 9 is comprised of four light receiving sections DET1-DET4 in first through fourth quadrants which are divided by two orthogonal division lines L1, L2, positioned adjacent to one another, and independent of one another. The focus error detector circuit 12 is connected to the quadrant detector 9. The quadrant photodetector 9 is positioned such that one of the division lines L1 is in parallel with a map in a direction in which the recording track of the optical disc 5 extends, i.e., in a tangential direction, and the other division line L2 is in parallel with a map in the radial direction. Respective opto-electrically transduced outputs from the light receiving sections DET1, DET3 symmetric about the center of the light receiving surface of the quadrant photodetector 9 are added by an adder 22, while respective opto-electrically transduced outputs from the light receiving sections DET2, DET4 are added by an adder 21. Outputs of the respective adders 21, 22 are supplied to a differential amplifier 23. The amplifier 23 calculates the difference between the supplied signals, and outputs the difference signal as a focus error signal (FES).
In this way, in the conventional focus error detector circuit 12, the outputs of the quadrant photodetector 9 are added by the adders 21, 22, respectively, and the difference between the outputs of the adders 21, 22 is calculated by the differential amplifier 23 to generate a focus error component. Specifically, as the signs of the light receiving sections on the quadrant photodetector 9 are indicated as their outputs, the focus error signal FES is expressed by the following equation (1):FES=(DET1+DET3)−(DET2+DET4)  (1)
A so-called sigmoid characteristic of the focus error signal (FES) is shown in FIG. 5. When focused, a light spot intensity distribution is symmetric about the center O of the light receiving surface on the quadrant photodetector 9, i.e., symmetric in the tangential direction and in the radial direction, so that a light spot in the shape of true circle, as illustrated in FIG. 3A, is formed on the quadrant photodetector 9. Therefore, the values derived by adding the opto-electrically transduced outputs from the light receiving sections positioned on the diagonals are equal to each other, resulting in the focus error component equal to “0.” On the other hand, when defocused, an elliptic light spot extending in a diagonal direction of the light receiving sections is formed on the quadrant photodetector 9 as illustrated in FIG. 3B or 3C, so that the values derived by adding the opto-electrically transduced outputs from the light receiving sections positioned on the diagonals differ in polarity from each other. Therefore, the focus error component output from the differential amplifier 23 presents a value in accordance with a focus error.
However, the astigmatic method is disadvantageously affected by noise introduced into the focus error signal (hereinafter called the “track traverse noise”) when a light beam spot traverses a track on an optical disc if an optical pickup has aberration such as astigmatism. In other words, even when focused as shown in FIG. 3A, FES=0 may not be resulted.
Unwanted astigmatism in an optical pickup device may occur when an alignment accuracy is low, for example, when light beam transmitting planes of optics such as a diffraction grating and a half mirror are tilted to and therefore are not perpendicular to the optical axis of an emitted light beam, or when the light beam emitted from a semiconductor laser itself has astigmatism. In addition, astigmatism occurs as well due to birefringence of a disc substrate which relates to irradiation and reflection of the light beam.
While such unwanted astigmatism can be eliminated by slightly canceling it using optics such as a shaping prism, a so-called oblique astigmatism component, which extends, for example, at an angle of 45° with respect to a direction corresponding to a tangential (track) direction or a radial direction to the astigmatism direction, remains in the entire optical system. For example, when a converged light beam is irradiated to a disc substrate made of polycarbonate (PC), astigmatism appears at 45° to the tangential (track) direction or the diagonal direction.
In the irradiation optical system and light detection optical system in the optical pickup device in accordance with the astigmatic method, optical elements (including a semiconductor laser as a light source, LED and so on) are designed to avoid introducing unwanted astigmatism. However, it is difficult to completely remove unwanted astigmatism in practice. With the existence of unwanted astigmatism not used for the focus servo, the track traverse noise is introduced in an attempt of generating a focus error signal from an optical disc having lands and grooves on an information recording surface thereof. This is because the light intensity distribution is uneven in the circular light beam spot on the quadrant photodetector 9.
In conventional optical pickups for CD, since an objective lens has a small numerical aperture NA and the focus depth is large, the noise does not cause problems even if it introduces more or less into the focus error signal. However, when information is read from an optical disc, such as DVD-RAM, which has lands and grooves, the FES noise included in a focus error signal will influence more gravely the focus servo of the objective lens because of a larger numerical aperture of the objective lens and a smaller focus depth. The influence becomes more grave if the depth of the grooves is set such that a push-pull error appears.
Further, as shown in FIG. 5, in the conventional astigmatic method, a sudden response characteristic is provided within a range in which an astigmatism difference occurs between the line image M including the minimum scattered circular image plane B and the line image S, i.e., in an effective range (capture range) of the focus error signal. It is desirable that an essentially ineffective focus error signal out of the capture range suddenly becomes zero. However, in the conventional focus error detection, the elliptic spot gradually becomes large due to defocusing, and extends off the detector, at which time the quadrant light receiving sections start outputting the opto-electrically transduced signals, and moreover, outputs from diagonal components leak in, a sudden characteristic is not achieved. As the objective lens has an increasingly larger numerical aperture corresponding to higher density optical discs in recent years, further limitations are imposed on the range of an operation distance of the objective lens. Therefore, there is a need for correct detection of the capture range in the conventional astigmatic method.
An attempt to correctly detect a capture range of focus servo is disclosed, for example, in Laid-open Japanese Patent Application No. 8-185635 entitled “Astigmatic method.” The disclosed method detects the capture range when a multi-layer disc is reproduced based on outputs of auxiliary detectors disposed outside of a quadrant photodetector. However, in this astigmatic method, an elliptic spot continuously becomes larger due to defocusing, and extends off the quadrant detector, at which time the quadrant detector starts outputting signals, thereby resulting in the inability to achieve a sudden capture range detecting signal characteristic. In addition, this astigmatic method is vulnerable to a shifted optical axis of a light beam spot to the quadrant photodetector. In the conventional focus error detection, the defocused light beam, spreading about the optical axis, will not largely extend off the photodetector. For this reason, for reproducing a multi-layer disc which has a narrow interlayer spacing such as DVD having a plurality of information recording surfaces stacked in the film thickness direction, the influence of interlayer crosstalk cannot be suppressed unless the area of the photodetector is set extremely small. A smaller area of a light receiving element will result in a smaller capture range, causing a deteriorated preability of a system.